Setting a default value for a tagged value type does not in fact record a default value for the tag. It displays the tag as if the value were set, but in fact the value is null, which shows up in RTF reports, etc.

This will display as "Term Type=Acronym" for a newly created element, but unless the user explicitly uses the drop-down to change the value, the reported value for the element is null. This is worse than useless, as the users are misled into believing that a default value exists.